Mega ST video output oddity

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Atarian Computing
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Joined: Tue Aug 22, 2017 4:27 am

Mega ST video output oddity

Post by Atarian Computing » Wed May 16, 2018 5:43 pm

One of my Megas, 2MB rev 5 has a weird problem. When I have a test cart inserted it displays everything just fine in test mode. Hi and low res. However, if I take the cart out it goes out of sync in Hi res. In low res without cart I can see a few bombs but kinda garbled.

I've tried many refurbed psus, I've exchanged the shifter, glue, mmu, and blitter for known good ones. Before I go further, does anyone have an idea where to look for next? And what causes the the output to be perfect with the cart? All tests are fine too.

Edit: Oh, and tried several monitors and adapters that are known to work fine.

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exxos
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Re: Mega ST video output oddity

Post by exxos » Wed May 16, 2018 9:39 pm

Sounds like loading on the ROM cart seems to help ? Probably change the SIL's for 2.2K... Had similar problem on my STE few weeks ago, machine would fail to boot when diagnostic cart inserted due to bus loading issues. Sounds like you got almost reverse problem to that.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

Atarian Computing
Posts: 98
Joined: Tue Aug 22, 2017 4:27 am

Re: Mega ST video output oddity

Post by Atarian Computing » Thu May 17, 2018 4:21 am

exxos wrote:
Wed May 16, 2018 9:39 pm
Sounds like loading on the ROM cart seems to help ? Probably change the SIL's for 2.2K... Had similar problem on my STE few weeks ago, machine would fail to boot when diagnostic cart inserted due to bus loading issues. Sounds like you got almost reverse problem to that.
Ah, good point. I'll have to order some and try. Thanks.

Atarian Computing
Posts: 98
Joined: Tue Aug 22, 2017 4:27 am

Re: Mega ST video output oddity

Post by Atarian Computing » Thu May 17, 2018 9:25 am

Ok, so I ordered a good bunch of these: http://uk.farnell.com/bourns/4609x-101- ... k%20bussed

Those are correct, right?

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exxos
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Re: Mega ST video output oddity

Post by exxos » Thu May 17, 2018 12:31 pm

Yep.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

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